1. Field of the Invention
The present invention relates to apparatus and method for controlling vehicular velocity of a host vehicle to follow a preceding vehicle which is running ahead of the vehicle (host vehicle), maintaining an inter-vehicle distance to the preceding vehicle substantially constant.
2. Description of the Related Art
A Japanese Patent Application First Publication No. Heisei 11-091 396 published on Apr. 6, 1999 exemplifies a previously proposed vehicular velocity controlling apparatus in which an acceleration/deceleration of the host vehicle is estimated in a case where the host vehicle is assumed to run on a flat (or horizontal) road according to an opening angle of an engine throttle valve, a shift position of a vehicular transmission, and a vehicular velocity during the vehicular run on a road segment having a gradient (or grade), a driven wheel velocity is differentiated to detect an actual acceleration/deceleration during the vehicular run on the road segment, a gradient resistance is estimated from the acceleration corresponding to a difference between the estimated acceleration and actual deceleration, and a vehicular driving force (braking force) is corrected on the basis of the estimated gradient resistance.
In a freeway, at a road segment on which a road gradient is largely varied, a vehicular velocity of each of the vehicles which are running on the road segment is varied so that a traffic density of the vehicle tends to become dense or thin and, thus, a traffic flow quantity is often reduced.
Such a road segment as described above is generally called a sag interval and a traffic congestion is frequently developed on the road segment.
On the other hand, a vehicle in which an automatic vehicular velocity control device (ASCD) to maintain the vehicular velocity constant is mounted becomes available on a market, it is expected that the vehicular velocity variation at the sag interval is suppressed to relieve the traffic congestion. However, as an actual practice, such an opportunity enabled to continue to run on the free way at a constant vehicular velocity is not so many and operation start and stop of the vehicular velocity control are repeated whenever the inter-vehicle distance to the preceding vehicle is adjusted. Therefore, an inter-vehicle distance controller (ACC: Adaptive Cruise Controller) to maintain the inter-vehicle distance to the preceding vehicle constant has been developed and has been put into practice on a market. However, since a control target is switched from the vehicular velocity control to the inter-vehicle distance control, the variation in the vehicular velocity becomes large with the influence of the gradient resistance received at the vehicular run on the sag interval. It is, therefore, necessary to compensate for the gradient resistance by some method in order to maintain such a preceding vehicle following characteristic as that of the vehicular run on the flat road even at the sag interval.
Although the previously proposed preceding vehicle following control apparatus described in the above-described Japanese Patent Application Publication has tried to solve such a problem as described above, the following inconveniences occur since the gradient resistance is compensated while the inter-vehicle distance control is continued.
That is to say, due to operational delays of the sensors and actuators, the previously proposed following control apparatus cannot obtain the estimated value of a correct gradient resistance at an inlet or outlet of the gradient road (ascending slope or descending slope) and the estimated value cannot immediately be reflected on the inter-vehicle distance control. Consequently, a large control error is temporarily developed.
For example, the vehicular velocity is temporarily decreased at a spot at which a road level is changed from the flat (horizontal) road segment to an ascending slope. When the plurality of vehicles are running in a file by means of the respective inter-vehicle distance controls, each inter-vehicle distance between the vehicles become narrowed due to a temporary deceleration of a most preceding vehicle and the following vehicles are accordingly decreased. Furthermore, further larger decelerations of the following vehicles occur due to the presence of the road gradient resistance. This phenomenon becomes more remarkable as an order of each of the following vehicle becomes trail. On the contrary, at a spot at which the road level is changed from the flat road segment to a descending slope, such a phenomenon as the large deceleration after the acceleration occurs as the order of the following vehicles becomes increased.
There is a possibility that the variation in the vehicular velocity in the inter-vehicle distance control not only gives an unpleasant feeling to vehicular occupant(s) but also induces a traffic congestion due to a development of a local roughness-and-fineness in a traffic flow density.
It is, hence, an object of the present invention to provide apparatus and method for controlling a vehicular velocity of a host vehicle to follow a preceding vehicle running ahead of the host vehicle which are capable of suppressing a vehicular velocity variation at a road segment on which the vehicle is to run and a road gradient is varied.
The above-described object can be achieved by providing an apparatus for controlling a vehicular velocity of a host vehicle to follow a preceding vehicle which is running ahead of the host vehicle, comprising: a vehicular velocity detecting section that detects the vehicular velocity of the host vehicle; an inter-vehicle distance detecting section that detects an inter-vehicle distance of the host vehicle to the preceding vehicle; gradient variation detecting section that detects a variation of a gradient of a road segment forward to the host vehicle on which the host vehicle is to run; control target switching section that switches a control target between an inter-vehicle distance control in which the vehicular velocity of the host vehicle is controlled to make a detected value of the inter-vehicle distance substantially equal to a target value of the inter-vehicle distance and a vehicular velocity control in which the vehicular velocity of the host vehicle is controlled to make a detected value of the vehicular velocity of the host vehicle substantially equal to a target value of the vehicular velocity of the host vehicle according to a result of detection of the variation of the gradient of the road segment by the gradient variation detecting section; a target vehicular driving force calculating section that calculates a target value of a vehicular driving force to be exerted by the host vehicle to achieve a content of the control target; and vehicular driving force controlling section that controls the vehicular driving force on the basis of the calculated target value of the vehicular driving force of the host vehicle.
The above-described object can also be achieved by providing a method for controlling a vehicular velocity of a host vehicle to follow a preceding vehicle which is running ahead of the host vehicle, comprising: detecting the vehicular velocity of the host vehicle; detecting an inter-vehicle distance of the host vehicle to the preceding vehicle; detecting a variation of a gradient of a road segment forward to the host vehicle on which the host vehicle is to run; switching a control target between an inter-vehicle distance control in which the vehicular velocity of the host vehicle is controlled to make a detected value of the inter-vehicle distance substantially equal to a target value of the inter-vehicle distance and a vehicular velocity control in which the vehicular velocity of the host vehicle is controlled to make a detected value of the vehicular velocity of the host vehicle substantially equal to a target value of the vehicular velocity of the host vehicle according to a result of detection of the variation of the gradient of the road segment; calculating a target value of a vehicular driving force to be exerted by the host vehicle to achieve a content of the control target; and controlling the vehicular driving force on the basis of the calculated target value of the vehicular driving force of the host vehicle.
This summary of the invention does not necessarily describe all necessary features so that the invention may also be a sub-combination of these described features.